This invention relates to a radio frequency transmission and receiving system and, more particularly, to such a system wherein a phase modulated pseudonoise sequence signal is received. The signal may be the radar reflection off a target from a signal transmitted by the receiving system itself or may be a signal transmitted directly from another location, for example, a ground landing beacon.
The present invention finds utility in applications where it is required that the amplitude of the strongest signal be determined accurately. Such an application may be, for example, in landing systems, air traffic control systems, and monopulse radar systems. In such a system, a radio frequency carrier is phase modulated by a pseudonoise code. The code consists of a sequence of M binary bits chosen to be pseudo random and to have low auto-correlation values. The relative phase of the modulated signal during a particular interval is +90.degree., depending on the sign of the code element during that interval.
After the signal is received, it must be processed to determine not only its time of arrival but also its magnitude at that time of arrival so that the strongest received signal can be determined. Also, many such contiguous measurements may sometimes be required from a scanning beacon so that an accurate bearing angle can be estimated by determining, through interpolation, the virtual maximum. Digital correlation provides a relatively low cost technique for determining the time of arrival of the received signal. However, such a technique does not readily lend itself to providing the high resolution and linearity of amplitude measurement required in the aforedescribed applications. Accordingly, it is an object of the present invention to be able to use digital correlation techniques and obtain amplitude measurement in a cost effective manner.